1. The Field of the Invention
The present invention relates generally to fluidized catalytic crackers and methods for upgrading hydrocarbons using novel fluidized catalytic crackers. More particularly, the present invention relates to a staged fluidized catalytic cracker that has a plurality of reactors that are fed fluidized catalyst in series and where an active stripper is positioned between each reactor to strip the fluidized catalyst between reactors.
2. The Related Technology
Catalytic cracking is the backbone of many refineries. It converts heavy feeds into lighter products by catalytically cracking large molecules into the smaller molecules that are found in highly valued transportation fuels. Catalytic cracking operates at low pressures, without significant hydrogen addition, in contrast to hydrocracking, which operates at high hydrogen pressures.
The most widely used process for catalytic cracking is fluidized catalytic cracking. In fluidized catalytic cracking (FCC), a catalyst, having particle sizes between about 20-100 microns circulates between a cracking reactor and a catalyst regenerator. In the reactor, hydrocarbon feed contacts hot, regenerated catalyst. The hot catalyst vaporizes and cracks the feed at 800-1100° F. (425-600° C.), usually 860-1040° F. (460-560° C.). The cracking reaction deposits carbonaceous hydrocarbons, which eventually turn to coke on the catalyst, thereby deactivating it. The cracked products are separated from the coked catalyst, usually with the aid of a catalyst stripper, and the stripped catalyst is then regenerated within the regenerator. A catalyst regenerator burns coke from the catalyst with oxygen containing gas, usually air. Regeneration of the catalyst by oxidation restores catalyst activity and simultaneously heats the catalyst to, e.g., 930-1650° F. (500-900° C.), usually 1110-1380° F. (600-750° C.). This heated catalyst is recycled to the cracking reactor to crack more fresh feed. Flue gas formed by burning coke in the regenerator may be treated for removal of sulfur oxides, nitrogen oxides, and particulates and for oxidation of carbon monoxide, after which the flue gas is normally discharged into the atmosphere. Catalytic cracking is an endothermic reaction. The heat for cracking and vaporization of the feed is supplied at first by the hot regenerated catalyst from the regenerator. In actuality, it is a portion of the feed that becomes coke which, when oxidized, provides the heat needed to crack the feed.
Catalytic cracking has undergone much development since its introduction over fifty years ago. The trend of development of the FCC process has been to use riser cracking with zeolite catalysts. Zeolite based catalysts of high activity and selectivity are now used in most FCC units. These catalysts have allowed refiners to increase throughput and conversion, compared to operation with amorphous catalysts. Use of zeolite catalysts effectively debottlenecked the reactor section, especially when a riser reactor was used.
Today, regenerators operate at very high temperatures. Most FCC units are heat balanced; the endothermic heat of cracking is supplied by burning the coke deposited on the catalyst. With poorer feeds, more coke is deposited on the catalyst than is needed for the cracking reaction. The regenerator operates at higher temperatures and excess heat is emitted in the form of high temperature flue gas. Regenerator temperature now limits many refiners in the amount of residue or high carbon residue feeds which can be tolerated by the unit. Special coolers to moderate the heat produced in regerators have become more common as deeper cuts of petroleum are fed to catalytic crackers.